An optical communication of digital signals is widely used in recent years, and a light emitting diode (hereinafter referred to as an “LED”) and a semiconductor laser are widely used as a light source. The semiconductor laser can carry out high-speed driving easily, but becomes unstable depending on temperatures. Therefore, in order to stabilize thermal operations, various devisals become necessary and a circuit configuration becomes complicated. Meanwhile, by using the LED, it is possible to realize the optical communication with a simple circuit configuration at low cost.
Incidentally, the optical communication is widely used for an in-car device in recent years, so that a wider range of operating temperatures has been required. A comparatively cheap plastic fiber is used for such optical communication. The transmittance of the plastic fiber becomes substantially maximal at wavelengths of 650 nm and 570 nm. A red LED is used, not an infrared emitting diode. That is, the transmittance of a quartz fiber is high at the wavelength of 1300 nm that is the wavelength of the infrared, and the quartz fiber is used for a high-speed communication. However, the quartz fiber is weak in bending, needs to be handled carefully, and is expensive. Meanwhile, as compared with the quartz fiber, the plastic fiber is more inexpensive and stronger in bending although the plastic fiber is lower in a decay rate. Therefore, the plastic fiber is used not for long-distance communication but for communication of several tens of meters. Moreover, due to a material characteristic of the plastic fiber, the transmittance of the plastic fiber becomes peak at the wavelength of 650 nm that is the wavelength of the red LED. Therefore, the plastic fiber has been generally used for the optical communication of digital audios, etc.
The following explains a method and circuit for satisfying demands of stabilizing the thermal operations and widening the range of operation temperatures.
As shown in FIG. 9, a conventional, basic LED driving circuit 100 for driving an LED 101 includes (i) a driving pulse current generating circuit 102 for generating a driving current Idriv for the LED 101 in accordance with a driving pulse signal Vin supplied from outside and (ii) a differentiating circuit 103 that is a peaking current generating circuit for generating a peaking current Ipeak obtained by differentiating a driving pulse signal Vin. The driving pulse signal Vin is supplied to the driving pulse current generating circuit 102 and the differentiating circuit 103 through an inverter 104. Then, a current Iled that is equal to the sum of the driving current Idriv and the peaking current Ipeak flows to the LED 101.
Incidentally, in the LED driving circuit 100, in the case of driving the LED 101 by the driving pulse current generating circuit 102 in accordance with the driving pulse signal Vin supplied from outside, a parasitic capacitor is provided between an anode and a cathode (not shown) of the LED 101 in a parallel manner. On this account, in the case of carrying out the high-speed driving of the LED 101, it is impossible to ignore a time it takes to charge and discharge the electric charge to and from the parasitic capacitor. Therefore, as shown in FIGS. 10(a) and 10(b), time delay occurs at rising and falling of a light pulse waveform. There are such problems, and in the case of using the LED 101, only a light pulse output depending on a specific response speed of the LED 101 can be obtained.
However, there are various proposals to carry out the high-speed driving whose speed is faster than the response speed of the LED 101. Among the proposals, there are, for example, a method called an overshoot method and a method called a backshoot method. The overshoot method is a method for reducing a rise time by adding the peaking current Ipeak and the driving current Idriv to quickly charge the electric charge to the parasitic capacitor of the LED 101. Meanwhile, the backshoot method is a method for reducing a fall time by applying a reverse bias to the LED 101 to quickly discharge the electric charge from the parasitic capacitor when the LED 101 does not emit light. Note that other examples of the LED driving circuit are disclosed in Japanese Unexamined Patent Publication No. 326569/2001 (Tokukai 2001-326569, published on Nov. 22, 2001), Japanese Unexamined Patent Publication No. 101047/2002 (Tokukai 2002-101047, published on Apr. 5, 2002), etc.
Incidentally, as shown in FIGS. 11(a), 11(b) and 11(c), in the conventional method for carrying out the high-speed driving of the LED, the amount of the peaking current Ipeak changes in accordance with a temperature change. The peaking current Ipeak decreases at high temperature, while the peaking current Ipeak increases at low temperature. The peaking current Ipeak changes because an output ON resistance of the inverter 104 increases at high temperature while it decreases at low temperature. Therefore, there are problems in that, as shown in FIG. 11(c), an adequately fast response cannot be obtained because the peaking current Ipeak becomes inadequate at high temperature.
Moreover, as shown in FIG. 11(b), the peaking current Ipeak becomes excess at low temperature, so that an overshoot is generated in a light output waveform.
Moreover, light outputs have respective temperature characteristics depending on the structure of the LED 101. Generally, the efficiency of light emission decreases at high temperature. On this account, it is necessary to obtain a constant light output by increasing the driving current Idriv. As with the above, the method used here is a method of superimposing the peaking current Ipeak on the driving current Idriv. Therefore, in the case of largely increasing the driving current Idriv, it is necessary to proportionally increase the peaking current Ipeak. However, the conventional method cannot arbitrarily adjust the peaking current Ipeak in proportion to the driving current Idriv. On this account, when the driving current Idriv is changed, the peaking current Ipeak becomes short or excess, and the light output waveform becomes a light output waveform W5 shown in FIG. 10(b). A light output waveform W4 (shown in FIG. 10(a)) that is required cannot be obtained.
Here, in the case in which a large overshoot is generated in a light signal transmitted in the optical communication, there is a method for detecting the peak of the overshoot at a circuit on a receiving end. In this case, a light receiving level is recognized at the peak, that is, the magnitude of the peaking current Ipeak is recognized as the level of the light, so that a malfunction occurs. Thus, in the case in which the overshoot of the light is large, a large error occurs in the light signal level, and deterioration of receiving sensitivity occurs.